Universal Time

[Jamie]

Are you confused by GMT and UTC? You're not sure whether to add or substract hours and what about daylight saving time? This will explain why this is all necessary and how to deal with time zones.

Universal Time

High Noon

The need for a “Universal Time” or even a time zone is a recent one. Up until the beginning of the nineteenth century when a trip of a few hundred miles could takes days to complete, there was little need to coordinate the time between different locations. Noon (which is from an Old English word for the “ninth watch”) was set simply as the time when the Sun was at its highest point in the sky. Som0

People were aware of the fact that as one moved eastward or westward this would change. Whether they believed that the Earth was rotating around its axis or that the Sun was revolving around the Earth, they understood that the Sun could not be high overhead England and China at the same moment. They also understood that if there was some way to determine the difference between high noon in England and high noon in China or in the middle of the Pacific Ocean they could determine their longitude – exactly how far east or west they are. All they needed was a clock that could accurately keep time as it was carried on a long ocean voyage. The British government offered a 20,000 pound prize for such a clock and after decades of work and several prototypes, John Harrison created the clock to do the job. Thanks to a version of such a clock, the great explorer James Cook was able to accurately map the South Pacific. In 1833 a time ball was hoisted up a flag pole every day at Greenwich Royal Observatory and dropped exactly at one o’clock so that ships passing on the Thames could set their clocks. They did this at one and not at noon because they would frequently make measurements at noon.

Time Zones and the Prime Meridian

But as railroad lines spread throughout the United Kingdom and the United States the time problem spread to land journeys also. Thanks to the railroad trains, people were able to travel at unprecedented speeds – 30 or 40 miles an hour. Someone would get on a train in New York and get off at Philadelphia and his watch would be off by 4 minutes because local time in New York was different than local time in Philadelphia.

The International Meridian Conference finally settled matters in 1884 when 25 delegations (one of them was from then independent Hawaii) voted to divide the world into 24 time zones. It also set the “Prime Meridian” which divides the world into East and West at the Royal Observatory in Greenwich. Before this, zero degrees longitude would vary on different maps, but thanks to British dominance of the seas and the important work that the Royal Observatory had done since its founding by Charles II in 1675 it became the location of the internationally recognized Prime Meridian. Furthermore, whenever it is necessary to have a common time recognized across international border for instance by seamen, pilots, radio operators and SkyWatch subscribers, the time at Greenwich is still used today. Nowadays instead of calling the standard time at Greenwich GMT it is called either UT (Universal Time) or UTC (Universal Coordinated Time).

But I Don’t Live In Greenwich

In order to convert UT to your local time, find your location on one of the web sites listed below:..

http://www.dxing.com/utcgmt.htm (U.S. and only)

http://time_zone.tripod.com/ (International)

...then add the value indicated. For instance if an event is listed at 17:00 UT time, I would see it in Israel (+2 hours) at 19:00 local time or 7PM. Keep in mind that converting from UTC can change the date. Let’s say you are located in Hong Kong (+8 hours) and you want to see an eclipse that is scheduled to start on the 15th of the month at 18:00 UTC. Adding 8 to 18 will give you 26:00 or 2AM the NEXT day – the 16th. If you are living in Los Angeles (-8 hours) and an occultation is scheduled at 03:00 on the 15th you would have to SUBTRACT 8 from 3 which would give you –5:00 or 7PM the previous day – the 14th. If daylight saving time is in effect you will have to add an additional hour

[The Warthog]

Good review, Jamie!

or UTC (Universal Coordinated Time).

What does DNA stand for?

[GordonCopestake]

Something OTHER than deoxyribose nucleic acid?

[MartinB]

"Did not attend" in health service parlance

[GordonCopestake]

Sounds like my short term memory after a night on the beer :|

[FLO]

Thanks Jamie

Perhaps it should be added to the glossary?

[The Warthog]

Something OTHER than deoxyribose nucleic acid?

DNA = National Dyslexic Association

[OXO]

:shock: